1. Field of the Invention
This invention relates to electronic systems, and more particular, for decoupling power distribution systems.
2. Description of the Related Art
In modern computers and other electronic systems, there is an ever-increasing demand for low-voltage, high-current power. Computer systems that draw 100 amperes of current at 1 volt are not uncommon. In order to make such low-voltage, high-current power distribution feasible, impedance in the power distribution system must be kept to a bare minimum.
In a power distribution system, inductance (and more particularly, inductive impedance) between the power source and the load is undesirable. Such inductive impedance can lead to higher levels of noise in the power distribution system, and this in turn can cause the release of electromagnetic energy that results in EMI (electromagnetic interference) or EMC (electromagnetic coupling). Power distribution system noise is a product of its impedance and its current. Thus, it is especially important to keep impedance at a minimum in today's computer systems, which often draw a high amount of current.
Inductive impedance in a power distribution system may be reduced by the use of decoupling capacitors. However, Equivalent Series Inductance, or ESL, (sometimes referred to as mounted inductance) may reduce the effect of providing decoupling capacitors. The ESL of a capacitor is that inductance associated with its mounting on a printed circuit board (PCB) or other circuit carrier. A capacitor's ESL is a function of both the internal capacitor characteristics and its mounting on the PCB (which includes various factors such as mounting geometry of the capacitor).
The impedance of a power distribution system is typically controlled using capacitance. Since the impedance provided by a capacitor is inversely proportional to its capacitance, the addition of capacitors between a power node and a reference (e.g., ground) node of a power distribution system reduces its overall impedance. However, the ability of capacitors to reduce the overall impedance of a power distribution system is limited. One factor is the ESL discussed above, as this is an inherent property of any capacitor. Another factor is the loop inductance, that is, the inductance in the current loop between power and ground (and which passes through the capacitor). Typically, capacitors are mounted on a surface of a printed circuit board (PCB) or other type of circuit carrier, while power and ground planes are located several layers below the surface. Surface pads to which the connectors are mounted are connected to the power and ground planes, respectively, through vias. The vias provide a vertical connection between the power/ground planes and their respective surface mounting pads. Since the inductive impedance is a distributed parameter, the impedance rises in proportion with the length of the current loop.